The adeno-associated virus (AAV) genome is built of single-stranded deoxyribonucleic acid (ssDNA), either positive- or negative-sensed, which is about 4.7 kilobase long. The genome comprises inverted terminal repeats (ITRs) at both ends of the DNA strand, and two open reading frames (ORFs): rep and cap. Rep is composed of four overlapping genes encoding rep proteins required for the AAV life cycle, and cap contains overlapping nucleotide sequences of capsid proteins: VP1, VP2 and VP3, which interact together to form a capsid of an icosahedral symmetry.
Recombinant adeno-associated virus (rAAV) vectors derived from the replication defective human parvovirus AAV2 are proving to be safe and effective gene transfer vehicles that have yet to be definitively identified as either pathogenic or oncogenic [3-4, 6, 18-19, 26, 31]. rAAV transduce non-dividing primary cells, are low in immunogenicity, and direct sustained transgene expression in vivo [6, 10, 20]. Infection with wild type AAV is associated with inhibition of oncogenic transformation and AAV inverted terminal repeats may actually confer oncoprotection [2, 28, 52-55]. A recent survey of panels of human tissues found that the marrow and liver were the two most common sites of naturally occurring AAV isolates in humans, suggesting that infection of marrow cells by AAV is not rare.
Use of viral vectors for gene therapy has been long considered. Due to its potential for long-lived correction and the ease of ex vivo manipulation, the hematopoietic system was one of the earliest targets of gene therapy. Despite significant effort, however, actual therapeutic success remains elusive [5]. This is due to the recognized inability of most viral vectors to efficiently transduce quiescent, non-dividing hematopoietic stem cells (HSC) [23] as well as safety concerns arising from insertional oncogenesis [15, 22]. However, stable gene transfer has been successfully demonstrated to both murine and human HSC by rAAV [8, 11-12, 24, 27, 29-30, 37].
It has been additionally difficult to effectively use viral vectors in gene therapy for treating neurological conditions, particularly central nervous system diseases or disorders due to the difficulty of crossing the blood-brain barrier, a cellular and metabolic separation of the circulating blood from the brain extracellular fluid created by tight junctions between endothelial cells that restrict the passage of solutes.
CD34 is cell surface glycoprotein and a cell-cell adhesion factor. CD34 protein is expressed in early hematopoietic and vascular tissue and a cell expressing CD34 is designated CD34+. Chromosomal integration of rAAV in human CD34+ HSC [8, 12, 16, 29] and efficient transduction of primitive, pluripotent, self-renewing human HSC capable of supporting primary and secondary multi-lineage engraftment has been demonstrated in immune-deficient NOD-SCID mice [29]. Transduction of primitive HSC capable of supporting serial engraftment was shown to be attributable to the propensity of rAAV to efficiently transduce primitive, quiescent CD34+CD38− cells residing in G0 [24]. Despite several reports of successful rAAV-mediated gene transfer into human HSC in vitro and in murine and non-human primate HSC in vivo, controversy regarding the utility of rAAV for HSC transduction still persists. These discrepancies arose primarily from short-term in vitro studies that assessed transduction by expression profiling and are attributable to the identified restrictions to transgene expression from rAAV2, including viral uncoating [35], intracellular trafficking [33], nuclear transport and second strand synthesis [36].
While AAV2 remains the best-studied prototypic virus for AAV-based vectors [1, 13, 18, 21], the identification of a large number of new AAV serotypes significantly enhances the repertoire of potential gene transfer vectors [14]. AAV1, 3 and 4 were isolated as contaminants of adenovirus stocks, and AAV5 was isolated from a human condylomatous wart. AAV6 arose as a laboratory recombinant between AAV1 and AAV2. Recently, more than 100 novel distinct isolates of naturally occurring AAV in human and non-human primate tissues were identified. This led to the use of capsids derived from some of these isolates for pseudotyping, replacing the envelope proteins of AAV2 with the novel envelopes, whereby rAAV2 genomes are then packaged using AAV2 rep and novel capsid genes. The use of novel capsids, the proteins as part of the viral shell, resulted in the circumvention of many limitations in transgene expression associated with AAV2 [32, 35-36].
In an effort to circumvent these restrictions, recent research has shown that novel capsid sequences result in reduced proteasome-mediated capsid degradation, increased nuclear trafficking and retention. Novel capsids, many of which utilize novel receptors, broadens the tropism of rAAV allowing for efficient transduction of previously refractory tissues and provides a means of circumventing highly prevalent pre-existing serologic immunity to AAV2, which posed major clinical limitations in a recent trial. Notably, some novel capsids appear to alter the intracellular processing of rAAV. For example, uncoating and transgene expression is accelerated in the context of AAV8 as compared to native AAV2 capsids. Recently, transgene expression was shown to be based upon capsid proteins, regardless of the serotype origin of the inverted terminal repeats (ITRs).
Naturally occurring AAV is readily identified in cytokine-primed peripheral blood stem cells. Capsid sequences of these AAV are unique. These capsids are capable of pseudotyping recombinant AAV2 genomes. Any improvement in the area of gene therapy regarding both permanent and reversible gene transfer and expression for therapeutic purposes, particularly if such advances targeted previously unsuccessfully targeted tissues like the central nervous system tissues including the brain, would be a significant improvement in the art. Moreover, safe and efficient gene delivery to stem cells remains a significant challenge in the field despite decades of research. Therefore the ability to genetically modify stem cells safely would represent a significant advance.